The main purpose of the respiratory system is to take in oxygen and eliminate carbon dioxide through the lungs. Oxygen, which is in the air, is breathed into the lungs (inhaled). Oxygen is necessary for the body to produce energy and sustain life. Carbon dioxide, a waste product of energy production, is dangerous if it builds up, and must be removed from the body by the lungs, back into air that is breathed out (exhaled). The respiratory system works closely with the circulatory system , which brings the oxygen from the lungs to the body's organs and removes carbon dioxide from the organs and takes it to the lungs.